1. Field of the Invention
This invention generally relates to DC power systems, and in particular to a DC-to-DC power supply which uses pulse width modulation (PWM) control.
2. Background of the Invention
While several methods of regulating a DC supply are known in the art, one such method uses pulse width modulation (PWM) control for its attendant advantages of accuracy of control and reliability. PWM control of power supplies is found in a wide variety of applications including AC voltage control, control of inverters and converters, voltage regulation and similar applications. PWM is used also in DC power supply control such as for instance in the voltage control of DC-to-DC power supplies. Regulated DC power supplies are indispensable to the operation of high speed processors, which has high transient loads.
PWM control per se, as used in different applications is known in prior art as exemplified for instance in U.S. Pat. No. 4,387,421 to Zach et al., and U.S. Pat. No. 5,986,426 to Rowan. Other examples of PWM control can be found in U.S. Pat. Nos. 4,626,979 to JaQuay, U.S. Pat. No. 5,777,503 to Richard A. Frank of Compaq Corporation, U.S. Pat. No. 5,929,606 to Richard A. Frank of Compaq Corporation, U.S. Pat. No. 6,088,490 to Iwata et al., U.S. Pat. No. 5,208,559 to Jordan, U.S. Pat. No. 5,534,889 to Daniel B. Reents et al., U.S. Pat. No. 5,272,327 to Nathan A. Mitchell et al., and U.S. Pat. No. 5,977,942 to Dale S. Walker et al. of Compaq Corporation.
In certain known systems of PWM control, a resistor (which may be an etched metallic member on a printed circuit board) is connected in series with an output inductor to provide adaptive voltage positioning. However, there is wastage of power and lack of control in such systems because of high I2R losses in the resistor and with its variation of resistance with temperature.
The V2 control technique uses a ripple voltage developed across an equivalent series resistance (ESR) of an output capacitor Cout as a ramp voltage in the PWM control loop. ESR is an inherent capacitor resistance. The ramp voltage is proportional to the AC current through the main inductor L1 and is offset by the value of the DC output voltage. This control scheme inherently compensates for variation either line or load conditions since the ramp signal is generated from the output voltage itself. However, using the output ESR resistance limits the transient response of the DC to DC converter. To improve the transient response, it is desirable to make the ESR resistance as small as possible. However, if the ESR resistance is too small there will not be sufficient ripple voltage generated to form the ramp signal for the V2 control technique. In accordance with the invention, a resistor and a capacitor coupled in series is placed across a square voltage waveform intermediate output of the DC power supply. Wherein the RC time constant of the resistor and the capacitor synthesizes the ramp signal from the square waveform.
In addition, in certain PWM control systems, a resistor is connected in series with an output inductor to provide adaptive voltage positioning. However, there is wastage of power and lack of control in such systems because of high 12R losses in the resistor. According to the embodiment, a current transformer is used to sense a primary current which is proportional to the output current. Thus, if the output current increases due to increase in transient load, the primary current increases proportionally. Conversely, if the output current decreases due to decrease in transient load, the primary current decreases proportionally. The primary current feeds into a resistor causing a voltage drop across the resistor. This voltage is fed into the feedback loop of the PWM control circuit. Thus when the voltage across the resistor increases, that voltage feeds into the feedback loop causing the output voltage Vout to decrease. Conversely, if the voltage across the resistor decreases, this voltage feeds into feedback loop and causes the output voltage Vout to increase. In this manner adaptive voltage positioning is obtained without having a resistor directly in the path of the output current.